In general, gas turbines are mainly provided with a compressor, a combustor, a turbine, and so on.
Among them, a known combustor is equipped with a plurality of kinds of fuel nozzles to achieve a reduction in the amount of NOx emission under a high load and combustion stability under a low load, for example, a DLN (dry low NOx) combustor (for example, see PTL (Patent Literature) 1).
Known examples of the above-described combustor are equipped with a main nozzle for premixed combustion and a pilot nozzle for diffusion combustion, and in addition to those, a top hat nozzle for premixed combustion designed to further reduce the amount of NOx emission.
The gas turbine is further provided with a fuel line that supplies fuel to the combustor, and the fuel line is configured to independently supply fuel to various kinds of fuel nozzles, such as the above main nozzle and pilot nozzle. On the other hand, the above fuel line is provided with various kinds of adjusting valve that regulate the pressure and flow rate of fuel to be supplied to the main nozzle and so on.
The above-described PTL 1 discloses an example in which fuel lines that supply fuel to the various kinds of fuel nozzles are each provided with a pressure adjusting valve, a flow-rate adjusting valve, a main nozzle, and so on, from the upstream side of the fuel flow.
Here, the pressure adjusting valve maintains a flow-rate-adjusting-valve differential pressure, that is, the difference between the pressure of fuel upstream of the flow-rate adjusting valve and the pressure of fuel downstream thereof, constant, and the flow-rate adjusting valve regulates the flow rate of fuel to be supplied to the main nozzle and so on disposed at the downstream side.
The flow-rate adjusting valve is controlled under constant differential pressure conditions, and the flow rate of the fuel is determined by calculation based on the flow-rate coefficient (Cv value) of the flow-rate adjusting valve. In the case of the invention disclosed in PTL 1, the flow of fuel in the flow-rate adjusting valve is always in a non-choked flow region. Therefore, a pressure term used in calculating the Cv value is expressed as a function of the pressure downstream of the flow-rate adjusting valve.
Here, the non-choked flow region is a region in which the pressure Pout of fuel at the outlet (downstream side) of the flow-rate adjusting valve and the pressure Pin of fuel at the inlet (upstream side) satisfy the following relationship:Pout>Pin/2
Specifically, the individual pressure adjusting valves are controlled so as to maintain constant differential pressures at the flow-rate adjusting valves provided in the individual fuel lines. On the other hand, the flow-rate adjusting valves are controlled on the basis of the actual measured fuel pressures and temperatures of fuel downstream of the flow-rate adjusting valves under constant differential pressures conditions and the degrees-of-opening calculated from the flow rates of fuel to be supplied to the main nozzle and so on, input from the outside.